Problem with creating a box on a cylinder
It good that you try to keep the geometry as simple as possible. But in this case you need more geometry to avoid pinching.

In the inner (curved) edge loop I used the "Loop Cut and Slide" tool and selected Even (E) offset. You can Flip (F) from where it's even. 



You can keep the other sections of the light sabre low in geometry. There are usually no reason to merge different parts seamlessly merged to each other.



You will find more tools and how you use them is up to you. I think that discover your way of solving problems is one of the more rewarding things with modelling. It's also quite tidies to do step-by-step instructions ;-) and at then end not that helpful.